The present invention relates to an electronic image registration system for a high-speed printer controller, and more particularly, to a dedicated hardware architecture for performing registration on an image in the form of a continuous stream of rasterized bit-map data. This registration is performed independently of page description language ("PDL") decoding and image rendering operations performed by the printer controller's graphics processing units.
The printing industry has advanced to the point that printing capabilities are no longer limited by the print engine speeds, but rather by the amount of processing time required by the printer controllers. Furthermore, as the printer controllers move into the field of variable data printing, in which each page map shares a common background or "template", and at least one set of image data bits changes for each page, the processing and dispatching times of such page-maps become increasingly significant. Therefore, for such printer controllers, it is imperative that the software-driven processing units on these printer controller boards have limited and specialized functions such that the PDL decoding, image processing and bit-map transport operations are performed as quickly and as efficiently as possible.
Therefore, on raster printer controller boards such as these, it is desirable to divide the controller architecture into a number of functional sections where each section is controlled by a dedicated microprocessor, operating in a pipeline fashion, and where each processing section is responsible for a specific functional operation of the raster printer controller.
High-speed printers utilizing these raster printer controllers can be configured to print variable image data on a predefined portion of a pre-printed form, i.e., printing customer addresses on a portion of a mass-mailing advertisement. As these forms are being printed, it is not uncommon that the printed images become misaligned with respect to the area in which they are to be printed on the form. Therefore, an image registration system is required to align or "register" the image with respect to this area on the form in real time. This misalignment of the image can be caused by circumstances such as the paper stretching or deforming, changes in the sizes of paper being used, etc.
One known method of registering the image with the pre-printed form is to mechanically shift the paper or mechanically shift the printing apparatus during the print job such that the image is aligned with the paper. Disadvantages with registering a print job in such a manner are that the mechanical registration can be complicated, imprecise and unreliable.
Another typical method for registering the image with the pre-printed form is to have the rendering units (the processing units which decode the PDL data and build the bit-map of the image from the decoded PDL data) on the printer controller render each image separately, incorporating the registration requirements in each rendering operation. A disadvantage with this method is that it significantly increases the complexity of the rendering operations and thus significantly slows the processing speeds of the rendering units, and in turn, of the printer controller.
Yet another known method for horizontally registering the image with the pre-printed form is to buffer a complete scan-line of the rendered bit-map data in memory and then shift the image as necessary. A disadvantage of this method is that it requires sufficient memory to hold an entire scan-line.
Thus, it is desirable to have a registration system which is performed electronically by the raster printer controller system; which is performed independently of the decoding and rendering operations of the controller board; which requires a significantly small amount of memory; and which is also substantially software independent, resulting in a very fast and reliable real-time registration operation.